1. Field of the Invention
The present invention is directed to a shape selective hydrocarbon conversion over a modified catalyst. The invention is also directed to a method for modifying the catalyst and the modified catalyst. A catalytic molecular sieve having protected acid sites is modified for shape selectivity by treatment with an amino silane polymer.
2. Description of the Prior Art
The term shape-selective catalysis describes unexpected catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g. by N.Y. Chen, W.E. Garwood and F.G. Dwyer, "Shape Selective Catalysis in Industrial Applications, 36, Marcel Dekker, Inc. (1989). Within a zeolite pore, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, oligomerization and aromatic disproportionation, alkylation or transalkylation reactions are governed by constraints imposed by the channel size. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react; while product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the zeolite pores or cages. A final type of selectivity results from configurational diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in selective toluene disproportionation to p-xylene.
Para-xylene is a product of xylene isomerization in commercial operations. Para-xylene may also be produced by methylation of toluene over a catalyst under conversion conditions. Examples are the reaction of toluene with methanol as described by Chen et al., J. Amer. Chem. Sec. 1979, 101, 6783, and toluene disproportionation, as described by Pines in "The Chemistry of Catalytic Hydrocarbon Conversions", Academic Press, N.Y., 1981, p. 72. Such methods typically result in the production of a mixture including para-xylene, ortho-xylene, and meta-xylene at their relative equilibrium concentration. A concentration of p-xylene above equilibrium concentration can be obtained depending upon the para-selectivity of the catalyst and the reaction conditions. The yield, i.e., the amount of feedstock actually converted to xylene, is also affected by the catalyst and the reaction conditions. The equilibrium reaction for the conversion of toluene to xylene and benzene proceeds as follows: ##STR1##
One method for increasing selectivity of zeolite catalysts is to modify the catalyst by treatment with "selectivating agents". Various silicon compounds have been used to modify catalysts which are in the hydrogen form to improve selectivity in hydrocarbon conversion processes. For example, U.S. Pat. Nos. 4,145,315, 4,127,616 and 4,090,981 describe the use of a silicone compound dissolved in an organic solvent to treat a hydrogen form of a zeolite. U.S. Pat. Nos. 4,465,886 and 4,477,583 describe the use of an aqueous emulsion of a silicone to treat the hydrogen form of a zeolite. U.S. Pat. Nos. 4,950,835 and 4,927,979 describe the use of alkoxysilanes to treat a hydrogen form of a zeolite. U.S. Pat. Nos. 4,100,215 and 3,698,157 describe the use of silanes to treat a hydrogen form of a zeolite.
There has been no suggestion, however, to modify a zeolite having protected acid sites which may therefore be modified in as-synthesized form, by contacting with an amino silane polymer to provide a zeolite with enhanced shape selectivity.
Accordingly, it is an object of the invention to improve selectivity in catalytic molecular sieves thereby improving shape selectivity in hydrocarbon conversion processes over the molecular sieves.